motor overheating with VFD, and incorrect amp readings
motor overheating with VFD, and incorrect amp readings
(OP)
We have a 350 HP fan driven by a VFD, the drive (saftronics GP10) has input 3% line reactor and DC choke, no reactor at the output. The motor always runs overheated, even though it has been running at 60 Hz for the past year or so. We want to try placing a reactor at the drive's output rather than at the input, to see if harmonics are causing this, but the electrical schematic states that the reactor should go either before or after the VFD, not in both. Is there any particular reason why you should't use reactors at both sides of a 350 HP drive? We have other similar but smaller drives with reactors at both sides, with no problems.
Something that dazzles us is that the drive's display always indicates a current 80-100 amps lower than the clamp-on true RMS ammeters we use. We have 3 identical 350 HP fan/vfd combinations close to each other, and on the 3 of them we have this difference in readings. We use GP10 drives extensively in other parts of the plant, 200 Hp or smaller, and the drive's display always agrees with the clamp-on ammeter.
By the way, motor to VFD distance is about 140 meters.
Any comments? we just changed a motor last week, and the new one is still overheating.
Something that dazzles us is that the drive's display always indicates a current 80-100 amps lower than the clamp-on true RMS ammeters we use. We have 3 identical 350 HP fan/vfd combinations close to each other, and on the 3 of them we have this difference in readings. We use GP10 drives extensively in other parts of the plant, 200 Hp or smaller, and the drive's display always agrees with the clamp-on ammeter.
By the way, motor to VFD distance is about 140 meters.
Any comments? we just changed a motor last week, and the new one is still overheating.





RE: motor overheating with VFD, and incorrect amp readings
Second, while reactors are nice cheap harmonic filters, they also drop voltage. For example, a 5% reactor drops 5% of its input voltage at rated current. Two 5% reactors will drop 10% of your supply voltage at full speed and load so it is likely that your motor would be starved for voltage with one input and one output reactor.
Third, you have rather long leads and should investigate some ringing and reflected wave protection for the motor. Even if the motor is inverter rated with excellent insulation, the insulation is probably in trouble.
Finally, are you sure the motor is actually too hot. Class F insulation is good for 110 degrees C at the insulation which would be about 100 degrees C on the exterior of the motor frame. Are you measuring the voltage with temp crayons or a heat meter?
RE: motor overheating with VFD, and incorrect amp readings
Thanks for your comments about the ammeter and reactors, we haven't taken that into account.
I understand that ringing and reflected wave prevention will reduce insulation failures, but I don't think it will drastically reduce temperature levels, right?
RE: motor overheating with VFD, and incorrect amp readings
IEC 60085 defines Class F as good for 155°C at the insulation, which might well equate to 130°C at the frame, perhaps even higher if there is good thermal contact.
A decent design shouldn't push the insulation right up to the theoretical maximum - many big machines are built using Class F materials but run at Class B limits (130°C) which gives enough margin to ensure a long service life and allows for the odd hot-spot.
Spot on comments about about exceeding the capability of Cat III protection on the instruments. So many people don't even consider the capability of their test gear.
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If we learn from our mistakes I'm getting a great education!
RE: motor overheating with VFD, and incorrect amp readings
Muthu
www.edison.co.in
RE: motor overheating with VFD, and incorrect amp readings
Now I'm curious about the differences between IEC 60085 and NEMA.
Can anyone explain the differences? Because, as ScottkyUK points out, that's a significantly large difference.
cokeguy, I don't think that reflected waves and ringing will affect the motor temp much. The temp question is separate.
RE: motor overheating with VFD, and incorrect amp readings
Double check the motor setup parameters. For example, make sure the motor voltage is set to the motor voltage (460V) and not the line utilization voltage (480V).
A good solution would be to get rid of the VFD. When you only run at 60Hz you are throwing away about 15kWh to 20kWh of electricity for every hour one of those fans is running.
RE: motor overheating with VFD, and incorrect amp readings
I would say the motor is not overheating based on the numbers measured at the outside frame surface.
RE: motor overheating with VFD, and incorrect amp readings
RE: motor overheating with VFD, and incorrect amp readings
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If we learn from our mistakes I'm getting a great education!
RE: motor overheating with VFD, and incorrect amp readings
Depending on the cable length to the motor is is posssibel for the voltage in the cable conductors to exceed 1500v assuming you rated volts are 480vrms
Neil
RE: motor overheating with VFD, and incorrect amp readings
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RE: motor overheating with VFD, and incorrect amp readings
RE: motor overheating with VFD, and incorrect amp readings
RE: motor overheating with VFD, and incorrect amp readings
RE: motor overheating with VFD, and incorrect amp readings
If client purchased a high efficiency or premium efficiency design typically the motor's full load speed increases by 5 to 10 RPM or more depending on how old the original motor were. IE: 1750 RPM (old) and 1770 - 1780 (new)
Application is a fan - so the extra 10 -- 20RPM increases the required horsepower (cube rule for fans?)